Sneaky surface hoar

Location Name: 
Forecaster Observation - Flathead Range
Observation date: 
Wednesday, December 16, 2020 - 16:30

Is this an Avalanche Observation: 
Yes
Observation made by: Forecaster

Tabs

Quick Observation

Today we went into the Flathead Range to assess storm totals and to suss out any potential persistent weak layers that may wake up with more snow and wind on the way.

  • Snow accumulations ranged from roughly 4 inches at mid elevations and 6 to 8 inches above 6000 feet.
  • We observed recent and active wind loading throughout the day at mid and upper elevations. At upper elevations, we were able to trigger wind slabs on small test slopes that were roughly 8 to 10 inches thick.
  • Snowpit test results in a treed area at mid elevations revealed nonpropagating results roughly 6 inches down. This failed within the storm snow. 
  • At upper elevations on a northeast aspect, we found a sneaky layer of surface hoar that was recently buried (12/14). Here extended column tests had propagating results with easy force. This layer is roughly 6 to 8 inches below the surface. For now...  
  • We had two large whumping collapses traveling an east/west ridgeline at roughly 7000 ft. 

Given these obvious signs of instability, the decision was easy to avoid avalanche terrain today. I broke a probe halfway through the day which reinforced this decision. We rode on terrain with slope angles less than thirty degrees and enjoyed every second of it. 

 

Snowpack, Avalanche, Weather Images: 
Snowpack, Avalanche, Weather Videos: 

December 16th in the Flathead Range

Travel Details
Region: 
Flathead Range - Marias Pass/Skyland
Activity: 
Skiing
Snowmobiling
Snowpack Details
Snowpack and Weather Details: 
Hide Terrain
Elevation of observation: 
5000-6500 ft
Above 6500 ft
Aspect(s) of observation: 
N
NE
E
SE
Red Flags: 
Avalanches from the past 2 days
Shooting cracks
Collapsing / whumpfing noises
Blowing snow
Persistent Weak Layers: 
Surface Hoar
Facets or Faceted Crust
Buried
New Snow in the past 24 hours: 
8.00in.
Total Snow Depth: 
140
More comments about the snowpack and weather: 
Surprised by the layer of surface hoar. Especially being a wind loaded area. ECTP3X2. 12/9 crust has 1 mm facets below and also propagated in test pits with medium force. We noticed the 12/14 freezing rain crust up to roughly 6500 feet. Here it was very friable and hard to find. I suspect it dissipates a little over 7k.
Blowing Snow: 
Moderate
Wind Speed: 
Light (Twigs in motion)
Wind Direction: 
Southwest
Air temperature: 
Below Freezing
Sky Cover: 
Overcast (OVC)
Highest Precipitation Rate: 
Very Light Snowfall (S-1)
Avalanche Details
Avalanche Details: 
Date and Time of Avalanche: 
Wednesday, December 16, 2020 - 12:00
Number of avalanches: 
1
Avalanche Type: 
Soft Slab
Failure Plane/Weak Layer: 
New/old snow interface
More information or comments about the avalanche: 

Triggered on a small wind loaded test slope. Initially I suspected it ran on crust. After revisiting the area and observing surface hoar, we feel confident that it failed on the 12/15 surface hoar layer. 

Hide Trigger
Trigger: 
skier
Trigger Modifier: 
Intentionally Triggered
Hide Terrain
Start Zone Slope Angle: 
38
Aspect: 
Northeast
Starting Elevation: 
above-treeline
Hide Size
Destructive Size: 
D1 Relatively harmless to people.
Relative Size: 
R1 Very Small
Crown Height: 
Less than 1 ft
Avalanche Length (Vertical Run): 
20ft.
Avalanche Width (Average width): 
15ft.
Avalanche Location: